Studies are being carried out on the renal excretion of urea in sheep fed diets low and high in protein. The specific question relates to the alteration of urea transport in the sheep nephron when dietary protein intake is varied, however, other aspects of renal function including blood flow, glomerular filtration and electrolyte excretion are being studied. In particular, urea excretion during saline and phosphate loading is being investigated. Other studies are being completed on the effect of uninephrectomy in the rat on renal concentrating ability with particular emphasis on the possible role of urea retention in the enhanced concentrating ability observed following uninephrectomy.